Thermal batteries are often used to power the circuitry in certain devices, such as missiles. Specifically, after activation, the battery powers, e.g., the electronic control circuitry and the motors that steer the missile.
A thermal battery is activated by igniting an internal portion of the battery. Upon ignition, the battery commences current and voltage production.
But unfortunately, upon activation, a thermal battery has a high equivalent series resistance (ESR)—typically on the order of tens of giga-ohms—which reduces the voltage level that the battery is able to generate across a load. Although the ESR reduces to a suitable value within a time typically on the order of ¼ to ¾ seconds, if the circuitry activates before the ESR is low enough—typically less than one ohm—and, thus, the output voltage high enough, the circuitry may initialize in an undesirable state or may otherwise malfunction. And if the circuitry malfunctions, it may cause a malfunction in the device, e.g., a missile, that incorporates the circuitry.
FIG. 1 is a schematic block diagram of a conventional device 10, which is a vehicle, such as a missile, having at least one load, such as a motor 20 and electronic circuitry 30, and a power source 35, including a thermal battery 40 with associated ESR 50. The motor 20 and electronic circuitry 30 are coupled to and receive a supply voltage Vs from the battery 40 via conductors 60 and 62. The assembly for igniting the battery is omitted for clarity.
Typically, the electronic circuitry 30 operates in a reset mode when the supply voltage Vs is between a minimum operational level and a reset level, e.g., 0.5 Volts (V), and is fully operational when Vs is greater than the reset level. But if while the circuitry 30 is fully operational Vs falls below the reset level, then the circuitry re-enters the reset mode. Unfortunately, the circuitry 30 re-entering the reset mode may delay the start-up time for the missile 10, or may cause the missile to malfunction.
More specifically, upon activation at missile-launch time, the battery 40 begins providing the supply voltage Vs to the motor 20 and electronic circuitry 30, which typically requires minimal current (on the order of a few milliamps) to reset itself, exit the reset mode, and perform, for example, pre-launch system checks. Consequently, because the circuitry 30 presents a relatively small load to the battery 40, Vs typically exceeds the circuitry's reset level relatively quickly, thus allowing the circuitry to become fully operational and perform the pre-launch routine within a few milliseconds after the battery 40 is activated. However, the motor 20, when operating, draws a relatively large amount of current on the order of 10 Amps, and thus presents a relatively large load to the battery 40. Therefore, if the circuitry 30 activates the motor 20 before the ESR 50 has fallen to a suitably small value, then the load presented by the motor 20 may cause Vs to fall below the reset level of the circuitry 30, which, as discussed above, causes the circuitry 30 to re-enters its reset mode. Unfortunately, the circuitry 30 re-entering its reset mode may undesirably delay or abort the launch of the missile 10.